The present invention relates to electronic communication systems, and more particularly to systems in which multiple signals are simultaneously transmitted at varying power levels.
In most electronic communication systems, it is often necessary that groups of information signals be amplified and transmitted simultaneously. For example, a cellular radio base station transmitter typically transmits signals to many active receiving mobile stations within a single geographic cell. Similarly, a satellite communications transponder amplifies and transmits large numbers of information signals destined for various participating remote stations. Because such systems customarily employ a frequency division multiple access (FDMA) scheme, in which information signals are modulated on signal carriers occupying contiguous frequency channels within an allocated frequency band, care must be taken to avoid inter-channel interference which may corrupt signal transmissions.
One possible source of such cross-channel interference is known as intermodulation distortion, which may result when two or more signals of different frequencies are mixed. For example, if two carriers of different frequencies are amplified using a non-linear amplifier, spurious outputs occur at the sum and difference of integer multiples of the original carrier frequencies. As described in detail below, third order intermodulation distortion products can present significant problems in FDMA systems. For example, a third order intermodulation product resulting from two relatively strong signals may disrupt transmission of a third relatively weak signal being transmitted on a carrier having a frequency equal to the frequency of the intermodulation product.
In power amplifiers, a trade-off is made between DC to RF power conversion efficiency and the level of intermodulation products generated by the amplifier. Thus, good DC to RF power conversion efficiency and high spectral purity can be contradictory requirements. The choice of amplifier is therefore significant in the design of the cellular base station architecture.
To date, there have been several base station architectures identified. Most commonly, base stations use a single carrier power amplifier (SCPA) with a frequency selective combiner. This architecture offers about 6-7% overall DC to RF power conversion efficiency due to the insertion losses encountered in the accompanying frequency combiner. The frequency combiner is also large and has "static" frequency selectivity which may need to be manually tuned during the base station installation.
Another common choice of architecture employs a multi-carrier power amplifier (MCPA). MCPAs are generally constructed to be highly linear in order to avoid generating intermodulation products which arise as a result of mixing the different modulated carrier frequencies within the amplifier. Therefore, even though no frequency combiner is required, this solution only offers an overall DC to RF power conversion efficiency of about 4-6%. Although comparable to the above mentioned SCPA/frequency combiner solution, the MCPA typically has much lower robustness and reliability. A high power MCPA is also a complex technology, i.e., not easy to master in production.
The intermodulation introduced by MCPAs has conventionally been reduced by using one of two methods: feed-forward cancellation amplification, or linear amplification with non-linear components (LINC). LINC amplification is quite complex and is currently completely unsuitable for low-cost, mass produced amplifiers.
A block diagram of a conventional feed-forward cancellation amplifier is illustrated in FIG. 1. In FIG. 1, an RF input signal is applied to coupler 100a which couples portions of the input signal to delay line 140 and to main amplifier 110. Main amplifier 110 produces an amplified output having intermodulation products generated due to non-linearities in main amplifier 110. A portion of the amplified output signal is coupled to summer 150 by coupler 100b. Delay line 140 delays the coupled portion of the input signal with respect to the output of main amplifier 110 producing a delayed signal such that the two signals reach summer 150 at approximately the same time. The output of summer 150 is an error signal which is coupled to auxiliary amplifier 160. Auxiliary amplifier 160 adjusts the amplitude of the error signal producing an error correction signal. The error correction signal should be matched in amplitude to the intermodulation products generated by main amplifier 110, but reversed in phase. The resultant vector cancellation of the intermodulation products is performed in coupler 100c where the error correction signal is subtracted from the amplified input signal. For the output signal to have intermodulation products which are greater than -60 dB down from the carrier frequencies, the vector cancellation must be performed with a high degree of accuracy. Typically this requires that the error correction signal be maintained with greater than 0.5 degrees phase accuracy and 0.1 dB amplitude accuracy which is difficult to achieve in production. The feed-forward technique can be used in an MCPA to effectively suppress intermodulation products but at the cost of low power efficiency and a high demand on complexity and component cost. In particular, high power MCPAs are difficult to master in production.
Accordingly, it would desirable to provide other techniques which reduce intermodulation distortion to, for example, compensate for non-linearities introduced by power amplifiers in multi-carrier environments.